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The Westermarck effect holds that being raised in close proximity puts a "biological stop sign" on romantic attraction. When close relatives separated during childhood are reunited, however, large numbers find themselves dealing with intense feelings.

When the first Israeli kibbutz (a word that roughly translates to "gathering") was established in 1909, it was obvious that this unique mix of Zionism and socialism had something special to offer. Who would have guessed that the movement, initially centered around collective farming, would shed light on one of the most interesting questions surrounding human sexuality, though?

Kibbutz kids were, while their parents were occupied with their respective tasks, raised collectively in age-based peer groups — forming much closer bonds than the children belonging to the same classroom do, bonds that more closely resembled the kind of relationship you would expect between siblings.

Later research of these children revealed that of 3,000 marriages within kibbutzim, only 14 sealed the unions between children raised within the same peer group — and none of those involved kids who spent their first six years of life together. This data represented the Westermarck effect in action. Finnish anthropologist Edvard Westermarck had argued as early as 1891 that it was being raised in close domestic proximity that put a biological "stop sign" on sexual attraction, a means to prevent incest and the possible genetical consequences. The effect, though not universal, has since been observed in other settings as well.

Yet, research suggests that humans tend to be sexually attracted to those who resemble them, and more specifically their opposite-sex parent, in a phenomenon known as assortative mating. Why that is the case isn't clear at the moment, but the connection exists. This doesn't mean Freud and his Oedipus complex were right, because, after all, the Westermarck effect plays a role as well, striking out close relatives as potential ideal partners.

What happens when close relatives, such as parent and child, or two siblings, were not together during those crucial formative years, though? What if these relatives, having been separated by circumstances like adoption, later meet up?

In a perfect storm of taboos, all kinds of factors can come together. There's the missing Westermarck effect, the proven tendency of humans to be attracted to those like them, an eerie and often spiritual-like feeling of recognition, and, so often, the shared pain of separation that only that other person can fill. And there you have it, a phenomenon that few people really like to ponder but that's nonetheless all too real and all too human: Genetic Sexual Attraction.

Mother of nine Monica Mares gave birth to her son Caleb when she was just 16. Finally meeting him 18 years later, she said: "It was love at first sight." We're all familiar with emotional reunions between birth parents and adopted children, people who finally get the chance to get to know each other after years, often decades, of yearning for each other. This story went a step beyond, though. The pair fell in love. Mares told the press:

"He is the love of my life and I don’t want to lose him. My kids love him, my whole family does. Nothing can come between us not courts, or jail, nothing. I have to be with him. When I get out of prison I will move out of Clovis to a state that allows us to be together. Caleb is willing to go through the same thing. Whatever it takes to be together."

Now charged with incest, a criminal offense, the two have been court-ordered to refrain from having contact with each other and are awaiting trial. Like others who have spoken publicly about Genetic Sexual Attraction, they were met by a wall of misunderstanding. It's "disgusting, it’s gross, she’s your mom", people told Caleb through his Facebook profile.

Falling in love with your parent, child, or sibling is certainly one thing: socially unacceptable. Given the images of coerced incest — rape — that everyone reading this right now is almost certain to be conjuring up, that makes all the sense in the world. We are not talking about unconsensual situations here, though, are we?